A hearing aid of this kind is described in DK patent application No. 1479/88, filed on Mar. 18, 1988, and in the corresponding U.S. application Ser. No. 322,387, filed on Mar. 13, 1989.
The hearing aid described as an exemplary embodiment in the applications referred to above is a so-called in-the-ear hearing aid, that comprises a vent canal communicating the part of the ear's external meatus situated internally of the hearing aid capsule with the ambient air. In such a hearing aid, the predominant sound-transmission path from the transducer to the primary microphone receiving the ambient sound to be amplified will normally comprise the vent canal, all other paths, such as more or less temporary gaps between the hearing-aid capsule and the wall of the meatus having a considerably greater attenuation than said path comprising the vent canal and hence also having a correspondingly smaller ability to produce unwanted feedback.
In hearing aids without such a vent canal, the predominance of one single sound-transmission path may be less marked; in fact, there may exist a number of possible paths, in which the predominance, i.e. the minimum attenuation, shifts from one to the other depending on the actual physical relations in the external meatus, being influenced by the user's jaw movements, such as in yawning, chewing or speaking and other conditions, such as the orientation in space of the user's head.
From the above it will be understood that with hearing aids thus having several possible predominant sound-transmission paths between the transducer and the primary microphone, the use of a single secondary signal path effecting feedback compensation will be insufficient to prevent feedback from occurring, when either of said several possible paths is made active, such as by the user yawning.